Your co-workers are people. As such they have thoughts, feelings, emotions, and desires, which guide them and affect them. Learning how people's psychology works, and how to motivate people and help them if they are feeling de-motivated - will help an employer be more effective, and make his employees more productive.
This is also the case for your customers, users, associates and friends, who you will surely have to interact with , make sure they are happy most of the time, and deal with their problems effectively.
Here are a few good resources on Psychology:
"Feeling Good: The New Mood Therapy" - the best book I've read about Psychology. A self-help book of Cognitive-behavioural therapy, this books explains what causes Clinical depressions, and other negative mood swings, and why people behave the way they do.
It is a stark anti-thesis to Freudian psychology, which makes little sense and is completely not helpful.
"The Neo-Tech Discovery" - an off-shoot of Ayn Rand' Objectivism, Neo-Tech is the best idea-system I've encountered yet. Note that it is easy to both dismiss Neo-Tech as a stupid cult, or to hugely misunderstand it at first. So when reading Neo-Tech go over the material (preferably without skipping, but possibly while taking breaks), and then let it sink for a while.
Note that as of May 2008, the old hyperlinks to the Neo-Tech site lead to redirects or PHP errors. You can still find the old pages on the web archive, and I hope they'll get fixed. In any case, it may be a good idea to order "The Neo-Tech Discovery" book. (It is not available in book stores).
"The Design of Everyday Things" - this book is a must read to everyone doing user-interface design. It explains how frustrating it is to use many everyday objects (and, by inflection, software and software devices), and how to design them right.
"Helplessness: On Depression, Development and Death" - I haven't read this book yet, but it was recommended on the "Joel on Software" book reviews, which I found to be of value. It is also written from the Cognitive-Behavioural Psychology viewpoint.
Naturally, becoming a more psychologically-capable person is a process. No one is fully friendly, tactful, helpful, and considerate at all times. But one must always aspire to be more.
For example, at one of the workplaces I had, whenever I told my boss that I didn't finish what he asked for, but instead found out something else of importance or made some other progress he said "So what you're saying is that you didn't achieve anything.". This is a classical case of the "All or Nothing Thinking" cognitive error, which is given in "Feeling Good" as a possible primary cause of depression.
This attitude caused me to think and feel that nothing I could do, would possibly please him, and that he was demotivating on purpose. This was one of the reasons that caused me to assume a variation of a clinical depression called "Hypomania", which in turn made me under-productive.